In general, it is known that when a transformer is energized without any load by being connected to a power source in a state where a residual magnetic flux is present in an iron core of a transformer, a large magnetizing inrush current flows. A size of this magnetizing inrush current is several times as large as a rated load current of the transformer. When such a large magnetizing inrush current flows, a system voltage fluctuates, and when the voltage fluctuation is large, a customer is influenced sometimes.
Consequently, it is known that as a method of suppressing the magnetizing inrush current, there is used a circuit breaker with a resistor in which a closing resistance and a contact are connected in series. The circuit breaker with the resistor is connected in parallel with a main contact of the circuit breaker. The circuit breaker with the resistor is closed prior to the main contact of the circuit breaker. Consequently, the magnetizing inrush current is suppressed.
Moreover, as another suppression method, there is known a method in which when a direct grounding system three-phase transformer is energized by three single-phase type circuit breakers, an optional circuit breaker for one phase is precedently closed, and then the remaining circuit breakers for the two phases are closed to suppress a magnetizing inrush current.
Furthermore, it is known that as a method of suppressing a magnetizing inrush current at a time when a non-effective grounding system three-phase transformer is energized by a three-phase collective operation type circuit breaker, a value of a magnetic flux remaining in an iron core at a time when the transformer is disconnected is measured, and a phase to be closed of the circuit breaker is controlled to suppress the magnetizing inrush current at a time when the transformer is energized.
On the other hand, as a method of converting a three-phase alternating-current voltage to single-phase alternating-current voltages, a Scott connection, a Woodbridge connection transformer, a modified Woodbridge connection and the like are known. These connection transformers are used, for example, when a power is supplied to a single-phase electric furnace, a single-phase alternating-current electric car, or the like.
However, the above-mentioned magnetizing inrush current suppression methods have the following problems.
In the magnetizing inrush current suppression method by the circuit breaker with the resistor, it is necessary to add the circuit breaker with the resistor to a usual circuit breaker, and hence the whole size of the circuit breaker increases.
Moreover, in any of the magnetizing inrush current suppression methods, it is not predicted that the above-mentioned transformer which converts the three-phase alternating-current voltage to the single-phase alternating-current voltage is introduced.
For example, in the method in which the residual magnetic flux is measured to control the phase to be closed of the circuit breaker, a control method for the three-phase transformer which is to be used in a power system cannot be applied, as it is, to the transformer which converts the three-phase alternating-current voltage to the single-phase alternating-current voltage. This is because in these connection transformers, even when a primary-side or secondary-side voltage is measured, the magnetic flux of the iron core of the transformer cannot be calculated as it is.